Gold is not such a great electrical conductor, whereas silver is, but gold is more malleable so can make a better physical connection.
Gold does not corrode. Copper and silver do.
I did put forward the scenario whereby Meridian do not output bit accurate from the disc. IIRC, CD's are 16 or 18 bit and spdif is 24 bit so there are 6 to 8 unused bits that Meridian can do something with - such as adding dither - people do like their 518's which can do similar.
I'd be surprised if they did this as it might make for incompatibility with 3rd party equipment. Resolution enhancement (a la 518) is handled in the G6x and 861 processors anyway.